Hexapods are parallel link mechanisms where six variable length struts act together to establish the fully constrained position of an end effector. They have been commonly employed to articulate flight simulators and increasingly in robotics and multi axis machine tools.
Many alternative solutions have been proposed, but all have necessarily been compromises, often resulting in mechanical complexity. The ideal geometry is an octahedron with three nodes at the top and three at the bottom and six struts forming successive triangles in between. In practice this geometry has only been possible where the struts vary their length telescopically between the node joints. This severely limits the ratio of maximum to minimum strut length and hence the articulation range of the hexapod.
Alternatively the struts can pass through the focal points of the nodes to change their effective length between the nodes. However, current designs require that the support frame must then support six individual nodes rather than three shared nodes. Additionally in order to ensure that the withdrawn struts cannot interfere with each other behind the frame, the nodes have to be sufficiently displaced from each other. For a given frame size this reduces the base length of the strut triangles, worsening the vertex positioning leverage. Also the base frame now supports six nodes so will be subject to more bending moments and calibration errors.
Struts have been successfully engineered using hydraulics or screw shafts—in particular ball screws for precise mechanisms. Ball screws however are expensive to make accurately and are generally massive, limiting their length and the speed with which they can be moved. They also do not assist in the application of a suitable length measuring transducer. Either the engineering becomes even more complex or you have to infer the length of the strut from the degree of rotation of the nut—not ideal.
Hexapods are naturally polar devices. As a result they find it easier to point their end effector towards the outside of their workspace, rather than inwards towards its center. It would certainly be an advantage if a hexapod demonstrated a more consistent tilting ability over more of its workspace. Hexapods are also 6 axis, although for machine tool use the common axis with the spindle serves little useful purpose (other than torque constraint). It would be helpful if this extra articulation could also be employed to extend the tilt range.
Hexapod machine tools have currently been packaged such that the workpiece has to be negotiated between either the struts or a similarly obtrusive machine frame. This does not permit ready vertical loading of potentially heavy parts.
Hexapods are six axis mechanisms, whereas strictly it is not necessary for a multi axis machine tool to articulate about more than five, since the sixth is in common with the axis of the spindle. It would save costs if only five servo powered struts were able to constrain the necessary five degrees of freedom.